Kotlin Variables and Data Types
A variable is a named storage box in memory. You put a value in it, give it a name, and use that name later in your code. Data types tell Kotlin what kind of value lives inside the box.
val vs var — The Two Ways to Declare Variables
Kotlin gives you two keywords for creating variables:
- val — the value cannot change after you set it (like a sealed envelope)
- var — the value can change anytime (like a whiteboard)
val country = "India" // Cannot change. country = "USA" would cause an error. var score = 0 // Can change. score = 10 is allowed.
When to Use Which
Situation | Use ---------------------------------|------ Store a birth date | val Track a game score | var Save the app version number | val Count items in a shopping cart | var
Always prefer val by default. Switch to var only when you know the value will change.
Kotlin Data Types
Every variable stores a specific kind of data. Kotlin has built-in types for numbers, text, and true/false values.
Number Types
Type | Size | Range | Example -----------|---------|-------------------------------|-------------------- Byte | 8-bit | -128 to 127 | val b: Byte = 100 Short | 16-bit | -32,768 to 32,767 | val s: Short = 500 Int | 32-bit | about -2 billion to 2 billion | val age: Int = 25 Long | 64-bit | very large numbers | val dist: Long = 9_000_000L Float | 32-bit | decimals (less precise) | val temp: Float = 36.6f Double | 64-bit | decimals (more precise) | val pi: Double = 3.14159
Text and Boolean Types
Type | Stores | Example -----------|----------------------|------------------------------- String | Text (words) | val name: String = "Riya" Char | Single character | val grade: Char = 'A' Boolean | true or false only | val isOnline: Boolean = true
Type Inference — Kotlin Guesses the Type
You do not always have to write the type. Kotlin looks at the value and figures it out automatically.
val city = "Mumbai" // Kotlin infers: String val age = 22 // Kotlin infers: Int val price = 9.99 // Kotlin infers: Double val active = true // Kotlin infers: Boolean
This makes your code shorter without losing safety.
Type Annotation — Writing the Type Explicitly
Sometimes you want to be specific. Write the type after a colon following the variable name.
val temperature: Float = 98.4f val itemCount: Int = 5 val username: String = "dev_rohan"
Diagram — Variable in Memory
Variable Declaration: val score: Int = 100 Memory Box: ┌───────────────────┐ │ Name: score │ │ Type: Int │ │ Value: 100 │ │ Mutable: No(val) │ └───────────────────┘
String Templates
You can embed a variable directly inside a string using the $ symbol.
val name = "Ananya"
val age = 20
println("My name is $name and I am $age years old.")
// Output: My name is Ananya and I am 20 years old.
For expressions inside strings, wrap them in curly braces:
val price = 50
println("After tax: ${price * 1.18}")
// Output: After tax: 59.0
Constants with const
Use const val for values that are fixed at compile time — meaning Kotlin replaces them with the actual value before the program runs. Constants must be at the top level or inside an object, and they must be a basic type like Int or String.
const val MAX_PLAYERS = 10 const val APP_NAME = "eStudy247"
Numeric Literals — Making Large Numbers Readable
Kotlin lets you use underscores inside numbers to improve readability.
val population = 1_400_000_000 // easier to read than 1400000000 val hexColor = 0xFF5733 // hex format for colors val binaryFlag = 0b1010_1100 // binary format
Type Conversion
Kotlin does not automatically convert one number type to another. You must do it explicitly.
val x: Int = 5 val y: Long = x.toLong() // Int → Long val z: Double = x.toDouble() // Int → Double val s: String = x.toString() // Int → String
This strictness prevents subtle bugs where a large number gets silently cut down to fit a smaller type.